1. Field of the Invention
The present invention relates to a DC-DC converter and an organic light emitting display using the same.
2. Discussion of Related Art
Recently, various flat panel displays having less weight and volume than cathode ray tubes (CRTs) have been developed. Flat panel displays include liquid crystal displays, field emission displays, plasma display devices, organic light emitting displays, etc.
Among others, organic light emitting displays generate images using organic light emitting diodes (OLEDs) that generate light by the recombination of electrons and holes generated corresponding to the flow of current.
Organic light emitting displays have various advantages, e.g., excellent color reproducibility and slimness so that its applications expand to various markets such as cellular phones, PDAs, MP3s, etc.
FIG. 1 is a circuit diagram showing a pixel in a conventional organic light emitting display. Referring to FIG. 1, the pixel is coupled to a data line Dm and a scan line Sn and includes a first transistor T1, a second transistor T2, a capacitor Cst, and an organic light emitting diode OLED.
A source of the first transistor T1 is coupled to a first power supply ELVDD, a drain of the first transistor T1 is coupled to an anode electrode of the organic light emitting diode OLED, and a gate of the first transistor T1 is coupled to a first node N1. A source of the second transistor T2 is coupled to the data line Dm, a drain of the second transistor T2 is coupled to the first node N1, and a gate of the second transistor T2 is coupled to the scan line Sn. A first electrode of the capacitor Cst is coupled to the first power supply ELVDD, and a second electrode of the capacitor Cst is coupled to the first node N1. An anode electrode of the organic light emitting diode OLED is coupled to the drain of the first transistor T1 and a cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS.
The voltage at the first node N1 of the pixel as described above is determined according to a data signal transferred through the data line Dm, and the first transistor T1 of the pixel drives a current from the first power supply ELVDD to the second power supply ELVSS according to the voltage of the first node N1. The organic light emitting diode OLED emits light according to the operation of the pixel, and a current having a magnitude represented by the following equation 1 flows through the organic light emitting diode OLED.
                              I                      o            ⁢                                                  ⁢            l            ⁢                                                  ⁢            e            ⁢                                                  ⁢            d                          =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    th                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    data                                    -                                      EL                    ⁢                    V                    ⁢                    DD                                    -                                      V                    th                                                  )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, Ioled represents the current flowing through the organic light emitting diode, VGS represents the voltage between the gate and source of the first transistor T1, Vth represents the threshold voltage of the first transistor T1, Vdata represents the voltage of the data signal, and ELVDD represents the voltage of the first power.
The organic light emitting display having the pixel as described above operates by receiving a power supply from a battery. Therefore, in order to extend a use time of the organic light emitting display, the battery may be desired to have a larger capacity. If the capacity of the battery becomes larger, the voltage of the power supply output from the battery may become higher.
The first power ELVDD and the second power ELVSS transferred to the pixel are generated by receiving the power supply from a DC-DC converter that includes a booster circuit and an inverter circuit. However, in a conventional booster circuit, the boosting range of the voltage has been limited. For example, in a case where the first power supply ELVDD has a voltage of 4.6V, if the input voltage is between 2.9V and 4.2V, the booster circuit generates a voltage of 4.6V. However, if the input voltage is 4.2V or higher, the booster circuit may generate a voltage higher than 4.6V.
Therefore, in the related art, the first power ELVDD has a voltage of 4.6V or higher, and thus a battery having a large capacity cannot be adopted.